A high speed optical modulator is a key component for applications of optical communications and on-chip interconnection. A silicon-on-insulator (SOI) based optical modulator is promising due to its compact footprint, compatibility with complementary metal-oxide-semiconductor (CMOS) technique, easy large scale integration with electronics, and thus resulting in a lower cost. Various kinds of silicon-based optical modulators have been demonstrated with various performances. However, for conventional optical modulators, one issue is how to increase the modulation speed and to reduce the operation energy without complicating the device structure and the fabrication process.
Furthermore, conventional silicon based optical modulators may have one or more of the following issues: (1) relatively high optical loss due to the waveguide implantation induced absorption loss; (2) relatively complicated implantation/doping scheme, with multiple implantations with different energy levels and dosages, to fabricate modulators for high-speed operation (e.g. associated parasitic capacitance and resistance); or (3) relatively small overlap between the optical field and the free carrier changing region, resulting in a low modulation efficiency. The term “free carrier changing region” may mean a depletion region which may be an insulating region within a conductive, doped semiconductor material where the mobile charge carriers (electrons and/or holes) have diffused away, or have been forced away by an electric field. The only elements remaining in the depletion region are ionized donor or acceptor impurities. A depletion region may be formed across a PN junction.